Exhaust gases to be emitted from an internal-combustion engine using gasoline as a fuel include harmful components such as total hydrocarbons (THC), carbon monoxide (CO), and nitrogen oxides (NOx). As such, it is necessary to purify each of the harmful components using a catalyst in such a manner that the total hydrocarbons (THC) are converted into water and carbon dioxide by oxidation, the carbon monoxide (CO) is converted into carbon dioxide by oxidation, and the nitrogen oxides (NOx) are converted into nitrogen by reduction.
As such a catalyst for treating exhaust gas (hereinafter, referred to as “exhaust gas purifying catalyst”), three way catalysts (TWCs) capable of oxidizing and reducing CO, THC and NOx have been used.
As such three way catalyst, a catalyst, in which a precious metal is supported on an inorganic porous material having a wide specific surface area, for instance, an alumina porous material having a wide specific surface area and then this is supported on a substrate, for instance, a monolithic substrate made of a refractory ceramic or metallic honeycomb structure or on refractory particles, is known.
The precious metal in this kind of three way catalyst has a function by which hydrocarbons in the exhaust gas are converted into water and carbon dioxide by oxidation, and carbon monoxide in the exhaust gas is converted into carbon dioxide by oxidation; meanwhile, nitrogen oxides in the exhaust gas are converted into nitrogen by reduction. Thus, it is preferable to maintain a ratio of fuel to air (air-fuel ratio) constantly (at the theoretical air-fuel ratio) in order to effectively provide a catalytic action with respect to both the reactions at the same time.
The air-fuel ratio of internal-combustion engines of automobiles and the like largely changes depending on driving conditions such as acceleration, deceleration, low-speed driving, and high-speed driving, and thus the air-fuel ratio (A/F) which fluctuates depending on the operating conditions of the engine is constantly controlled by using an oxygen sensor (zirconia). However, it is not possible for the catalyst to sufficiently exert performance as a purifying catalyst only by controlling the air-fuel ratio (A/F) as described above, and thus the catalyst layer itself is also required to have an action of controlling the air-fuel ratio (A/F). Hence, a catalyst obtained by adding a promoter to a precious metal which is a catalytically active component is used for a purpose to prevent a decrease in the purification performance of the catalyst caused by the change in the air-fuel ratio by the chemical action of the catalyst itself.
As such a promoter, a promoter (referred to as “OSC material”) having oxygen storage capacity (OSC) to release oxygen in a reducing atmosphere and to absorb oxygen in an oxidizing atmosphere is known. For instance, ceria (cerium oxide, CeO2) or ceria-zirconia composite oxide is known as the OSC material having the oxygen storage capacity.
Ceria (CeO2) has a characteristic that extends a range (window) of the air-fuel ratio capable of efficiently purifying CO, THC, and NOx, and in which desorption and absorption of attached oxygen and lattice oxygen in the cerium oxide can be performed depending on a level of an oxygen partial pressure in the exhaust gas. In other words, when the exhaust gas has a reducing property, the cerium oxide desorbs the oxygen (CeO2→CeO2-x+(x/2)O2) to feed the oxygen into the exhaust gas, thereby causing an oxidation reaction. On the other hand, when the exhaust gas has an oxidizing property, the cerium oxide reversely takes the oxygen in oxygen deficiency (CeO2-x+(x/2)O2→CeO2), thereby reducing an oxygen concentration in the exhaust gas to cause a reduction reaction. In this way, the cerium oxide fulfills a function as a buffer that decreases a change in the oxidizing and reducing properties of the exhaust gas, and has a function of maintaining the purification performance of the catalyst.
Further, the ceria-zirconia composite oxide that causes zirconia to be dissolved in this ceria is added to many catalysts as the OSC material, because the oxygen storage capacity (OSC) thereof is further excellent.
The ceria-zirconia composite oxide is compatible with the precious metal, and therefore, when the precious metal is supported on the ceria-zirconia composite oxide, an anchoring effect is exerted and the ceria-zirconia composite oxide is hardly sintered even a thermal load is applied thereto, thereby dispersibility can be maintained. Thus, the ceria-zirconia composite oxide is used as not only the OSC material but also a carrier of the precious metal.
However, since the ceria-zirconia is a rare earth and expensive, in order to provide an inexpensive catalyst, there has been a limit to use the ceria-zirconia composite oxide as the carrier of the precious metal.
Therefore, a catalyst, in which the OSC material such as ceria or ceria-zirconia composite oxide is used as the carrier of the precious metal along with alumina, and the catalyst purification performance can be inexpensively and stably exerted, has been hitherto disclosed.
For instance, Patent Document 1 (JP H06-226096 A) discloses an exhaust gas purifying catalyst which comprises a first catalyst layer formed on the surface of a catalyst carrier and containing Pd and alumina, and a second catalyst layer formed on the outer surface of the first catalyst layer and containing Pd and ceria.
Patent Document 2 (JP H10-296085 A) discloses an exhaust gas purifying catalyst having two or more catalyst coat layers on a catalyst substrate, wherein an upper layer of the catalyst coat layers comprises a refractory inorganic oxide mainly composed of activated alumina, at least one element selected from the group of Ce, Zr and compounds thereof, and Rh, and a lower layer of the catalyst coat layers comprises a refractory inorganic oxide mainly composed of activated alumina, and Ce, Zr, Pd and Ba.
Patent Document 3 (JP 2001-79404 A) discloses an exhaust gas purifying catalyst which comprises a heat-resistant support carrier, a first coating layer which is directly supported and formed on the surface of the heat-resistant support carrier, and a second coating layer formed on the first coating layer, wherein the first coating layer includes palladium and alumina, and the palladium is supported on a part or whole of the alumina, and the second coating layer includes at least one of platinum or rhodium, and a zirconia-cerium composite oxide and at least either the platinum or the rhodium is supported on a part or whole of the zirconia-cerium composite oxide.
Patent Document 4 (JP 2002-11350 A) discloses an exhaust gas purifying catalyst, in which a first coating layer is formed on a heat-resistant support carrier and a second coating layer is formed on the first coating layer, wherein the first coating layer contains alumina on which palladium is supported, and the second coating layer contains a Ce—Zr-based composite oxide on which platinum and rhodium are coexistently supported and a Zr—Ce-based composite oxide on which platinum and rhodium are coexistently supported and the composition thereof is different from the aforementioned composite oxide.
Patent Document 5 (JP 2013-184125 A) discloses an exhaust gas purifying catalyst which comprises a Pd catalyst layer containing Pd particles and a Rh catalyst layer containing Rh particles formed on the Pd catalyst layer, wherein a Pd carrier for supporting the Pd particles in the Pd catalyst layer is a composite oxide containing Al2O3 as a main component and ZrO2.
Patent Document 6 (JP 2013-220401 A) discloses an exhaust gas purifying catalyst which has a catalyst layer composed of an upper layer and a lower layer provided on a substrate. The upper layer contains Pt and Rh as a catalyst metal, and an oxide carrier supporting the catalyst metal, and has a CeZr-based composite oxide and an alumina composite oxide containing La as the oxide carrier, in which the Pt of the upper layer is supported only on the alumina composite oxide containing the La, and the Rh of the upper layer is supported on the CeZr-based composite oxide and the alumina composite oxide containing the La respectively. The lower layer contains Pd but not Rh as a catalyst metal, and has a CeZr-based composite oxide and an alumina composite oxide containing La as an oxide carrier supporting the catalyst metal, in which the Pd of the lower layer is supported on the CeZr-based composite oxide and the alumina composite oxide containing the La respectively.
Patent Document 7 (JP 2014-117701 A) discloses a gas purifying catalyst for an internal-combustion engine which includes a carrier and a catalyst layer formed on the carrier, wherein the catalyst layer has a first catalyst including a first support containing alumina and Pd supported on the first support, and a second catalyst including a second support containing a ceria-zirconia composite oxide and Rh supported on the second support.
Patent Document 8 (JP 2014-144426 A) discloses an exhaust gas purifying catalyst which comprises a first catalyst layer including Pd, an oxygen storage capacity (OSC) material, and an inorganic porous material, and a second catalyst layer including Pt or Rh or both of them and an inorganic porous material on a substrate, wherein, in the catalyst comprising a structure in which the second catalyst layer is disposed on the upper side of the first catalyst layer, the first catalyst layer contains one or two or more transition metals selected from a group consisting of Co, Ni, Mn, Cu, and Fe.
Patent Document 9 (WO 2014/156676) discloses a catalyst structure which comprises a substrate, an upper catalyst layer, and a lower catalyst layer, wherein the upper catalyst layer is a porous layer containing an inorganic porous material formed from an oxide whose crystalline structure belongs to an apatite type, and the lower catalyst layer is a porous layer containing an inorganic porous material having an oxygen storage/release capacity function (OSC function).
Incidentally, an exhaust gas purifying catalyst for a saddle-riding-type vehicle such as a two-wheeled automobile has special problems different from those of an exhaust gas purifying catalyst for a four-wheeled automobile. For instance, in comparison with the exhaust gas purifying catalyst for the four-wheeled automobile, the exhaust gas purifying catalyst for the two-wheeled automobile is required to have a small capacity and yet to exert a high purification capacity because of a limited space where the catalyst is mounted.
In this way, the exhaust gas purifying catalyst for the two-wheeled automobile has the special problems different from those of the exhaust gas purifying catalyst for the four-wheeled automobile. As such, the following proposals are made with respect to the conventional exhaust gas purifying catalyst for the two-wheeled automobile.
For instance, Patent Document 10 (JP 2001-145836 A) proposes an exhaust gas purifying catalyst that contains a metallic carrier comprising a cylindrical punching metal made of heat-resisting stainless steel, an undercoating layer of heat-resistant inorganic oxide to which an oxygen occlusion material placed on a surface of the carrier is added, and a catalyst supporting layer on which the catalyst placed on the undercoating layer is supported, as an exhaust gas purification catalyst that suppresses detachment of a catalyst layer caused by heat or vibrations to the minimum extent, suppresses thermal degradation of a catalytic material as well, and exerts good purification performance under severe conditions, and that uses cerium oxide or a composite oxide of cerium and zirconium as the oxygen occlusion material.
Patent Document 11 (WO 2010/109734) discloses an exhaust gas purifying catalyst for a saddle-riding-type vehicle which has a first catalyst layer formed on a surface of a carrier and a second catalyst layer formed on the first catalyst layer, wherein the first catalyst layer has a carrier which is formed from a composite oxide comprising 45 to 70% by mass of CeO2, 20 to 45% by mass of ZrO2, 2 to 20% by mass of Nd2O3, and 1 to 10% by mass of La2O3, and a catalyst component comprising a supported metal Pd or a Pd oxide, and the second catalyst layer has a carrier which is formed from a composite oxide comprising 50 to 95% by mass of ZrO2, 0 to 40% by mass of CeO2, 2 to 20% by mass of Nd2O3, and 1 to 10% by mass of La2O3, and a catalyst component comprising a supported metal Rh or an Rh oxide, or alternatively a catalyst component comprising a supported metal Rh or an Rh oxide and a catalyst component comprising a metal Pt or a Pt oxide.
Furthermore, Patent Document 12 (JP 2013-208578 A) discloses a palladium monolayer catalyst for an exhaust gas to be emitted from a saddle-riding-type vehicle which comprises a substrate as the exhaust gas purifying catalyst for a saddle-riding-type vehicle, and a catalyst layer, that has the form of a monolayer, which contains palladium as a catalyst active component, an inorganic porous material as a catalyst carrier, ceria (CeO2) particles as a promoter component, and barium.